1. Technical Field
The present invention relates to a method and apparatus for tunnelling data in a network. In particular, but not exclusively, the invention relates to an optical communications network such as synchronous digital hierarchy (SDH) networks or synchronous optical network (SONET) or an optical transport network.
2. Related Art
SDH and SONET are standards for telecommunications networks. SDH is an International Standard. It is used throughout Europe. It is also used in other countries for international connections. SONET has been standardised for use in the US and Japan. The present invention is applicable to both SDH or SONET. Communication networks for transporting SDH or SONET traffic are well known. SDH and SONET have become the dominant formats for transport in both national and international networks. In Europe, for example, as demand for bandwidth has increased, SDH has kept pace, with a capability to support client bit rates that span five orders of magnitude, from 1.5 Mbit/s to about 40 Gbit/s. The SDH and SONET networks are not described in greater detail here, as the present invention is compatible with existing SDH and SONET networks. However, where necessary for the present invention to be understood, further details are given below. Where transmission and formatting occurs with reference to a particular standard, these standards are referred to in the description. Prior to transmission of data over an SDH or SONET network, the data is formatted into a frame. The SDH or SONET frames last 125 microseconds. There are thus, 8000 frames per second. The frame includes an overhead area, which includes management data and a payload area (also known as a virtual container capacity) which includes the data for transmission. As the SDH or SONET frames are transmitted through the SDH or SONET network, respectively, all or certain parts of the overhead area are accessed by various network elements. At the network nodes the data contained in the overhead area is converted from the optical domain to the electrical domain. The network nodes process the electronic overhead data using standard digital processing techniques. The payload data is transmitted unaffected by the network elements through the network to a network termination equipment depending on the management information contained in the overhead area. Depending upon the type of processing point, eg. regenerator, multiplexer, demultiplexer, add-drop multiplexer, digital cross connect, etc, some or all of the overhead area may be accessed.
A problem with the present SDH and SONET frame structures is that because data contained in the overhead area may be accessed at network elements, it is not possible to transparently transport SDH or SONET frames over an SDH or SONET network, i.e. it is not possible to transport complete SDH or SONET frames over their respective networks intact without data in the overhead area being accessed, and in many circumstances removed or replaced.